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Solutions are homogeneous mixtures resulting from the dissolution of solutes in solvents. Solution concentration is quantified using molarity, normality, or molality, with molarity being the most common. Molality is useful for colligative properties as it is weight-based and not temperature-dependent.
Solutions are homogeneous mixtures that result from the dissolution, at the molecular level, of one or more ‘solutes’ within the ‘solvent’ – the dissolution medium. The solvent can also consist of several substances, as long as they too dissolve in each other. In ordinary usage, the word solution refers to substances dissolved in a liquid solvent, although the broad usage of the word is not so limited. Scientists call the amount or richness of solute within a solution its solution concentration. There are several ways to quantify or assign a numerical value to the concentration of the solution.
The methods used to report concentration vary depending on whether the usage is scientific or not, and sometimes depending on the particular science. Especially among analytical chemists, the most common unit for reporting solution concentration is “molarity.” This term comes from the word “mole,” referring to the molecular weight of a particular compound in grams. It can easily be seen that because their molecular weights differ, one mole of sugar is not equal in weight to one mole of salt.
Consider how a monomolar solution of table salt is prepared. Sodium chloride has the chemical formula NaCl: it is the reaction product produced by combining sodium metal (Na) with chlorine gas (Cl). The atomic weight of sodium is 22.99; the atomic weight of chlorine is 35.45. Simple addition gives the molecular weight of the salt as 58.44, i.e. one mole of NaCl weighs 58.44 grams. Dissolving this amount of NaCl in water to make one liter (1.06 quarts) of solution yields exactly one molar (1.0 M) solution.
Less frequently, the concentration of the solution can be expressed in terms of “normality” or “molality”. The definition of normality is not very different from that of molarity, but incorporates the concept of “equivalents”. For example, a solution that is 1.0 molar in phosphoric acid (H3PO4), since it produces three hydrogen ions for each molecule of phosphoric acid, is normal 3.0 (3.0 N). Although at first it may seem advantageous to use normality instead of molarity as the concentration standard of the solution, normality is not an absolute term, but depends on the use of the solution. For this reason, the International Union of Pure and Applied Chemistry has called for the cessation of normality in expressing the concentration of the solution.
Molality is used even less often than normal. A solution is a molal (1.0 m) if it consists of one mole of solute dissolved in one kilogram – not of solution – but of solvent. At first, it might seem that molality does not offer particularly valuable properties that make it useful as a term for solution concentration. However, it is not about volume, just about weight, for both solute and solvent. This means that molality does not depend on temperature, making it the unit of choice in those areas of chemistry involving “colligative” properties – those properties involving particle number.
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